Fitness and Selection

Abstract

Selection affects the genetic composition of a population through differences in fitness amongst genotypes. With discrete
generations, fitnesses are usually measured relative to the value for some standard genotype because only relative fitnesses
matter. For an autosomal locus with two alleles, when the highest fitness is associated with homozygotes for a particular
allele, this allele will spread through the population. When the heterozygote has the highest fitness, a polymorphism is maintained
by selection. Variation can also be maintained by frequency‐dependent selection, temporal and spatial variation in fitnesses
and a balance between the input of deleterious mutations and their elimination by selection. Selection theory can be extended
to more complex situations such as overlapping generations. Selection can act at different levels, including selection among
gametes produced by the same individual or amongst groups of individuals. Selection at one site in the genome can also influence
evolution and variation at linked sites.

Heterozygote advantage at a single locus with two alleles, A1 and A2. The Y axis shows the change in frequency (Δq) of A2, as a function of its frequency in the population, q (X axis). The red arrows show that the change in frequency is always in the direction of the equilibrium frequency, q*, whose position is indicated by the vertical blue line.

Figure 2.

The loss of a rare beneficial mutation from a population, as result of random sampling events. Each red circle indicates a
copy of the initial mutation, which arose in generation 0. The numbers at the bottom indicate the numbers of generations after
the origin of the mutation.

Figure 3.

The effects of a selective sweep (a) and background selection (b) on linked variants in the absence of recombination. Each
horizontal line represents a different copy of the same chromosome in the population. Black circles indicate neutral variants,
white circles beneficial mutations and red circles deleterious mutations.

References

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